Diagnosis of acute myocardial infarction (MI) and unstable angina in the emergency department (ED) is important because the current standard of care is immediate thrombolytic therapy or coronary angioplasty for patients with acute MI, and intravenous heparin for patients with unstable angina. In addition to early reperfusion of ischemic myocardium, sustained patency of the infarct artery is an important determinant of survival in patients with acute MI. Thus, even when early treatment ifn the ED is initially successful in achieving reperfusion, subsequent reocclusion of the infarct artery following admission to the hospital may negate the entire beneficial effects of these early treatments. Therefore, it is imperative to find better ways to monitor a patient for ischemia in the ED as well as throughout a patient's hospitalization in critical care and "step-down" telemetry units. Reports suggest that continuous ST segment monitoring of the standard 12-lead ECG may improve detection of ischemia; however, such monitoring is difficult to accomplish because it requires multiple electrodes placed in locations that interfere with patient care and that create a noisy signal with body movement. The goal of this research is to test whether continuous ST segment monitoring using a simple lead system (derived 12-lead ECG) is comparable to the currently-accepted "gold standard" lead system (standard 12-lead ECG) for detecting ischemia and MI. The experimental method involves mathematically deriving 121 ECG leads from 3 base signals recorded from the patient. A prospective, comparative, "within-subjects" design will be used in which 685 patients with acute MI or unstable angina will undergo continuous 12-lead ST segment monitoring of both experimental and standard ECGs from the time they arrive in the ED until they are discharged from the hospital. Experimental and standard ECG methods will be compared for diagnostic accuracy for presence-absence of MI, infarct location, magnitude of ischemia ("total ischemic burden"), detection of delayed- onset ischemia in initially non-diagnostic ECGs, and recurrent ischemia during hospitalization for MI or unstable angina. Cardiac enzymes, echocardiographic, angiographic, and myocardial perfusion imaging data will be used as an independent standard relative to a correct diagnosis. Statistical analysis will include calculation of the sensitivity, specificity, and predictive accuracy of the experimental method, which will be compared to the standard ECG method.